Faculty Publications
Permanent URI for this communityhttps://idr.nitk.ac.in/handle/123456789/18736
Publications by NITK Faculty
Browse
46 results
Search Results
Item Electronic band structure and photoemission spectra of graphene on silicon substrate(SPIE spie@spie.org, 2014) Javvaji, B.; Ravikumar, A.; Shenoy, B.M.; Roy Mahapatra, D.; Rahman, M.R.; Hegde, G.M.Synergizing graphene on silicon based nanostructures is pivotal in advancing nano-electronic device technology. A combination of molecular dynamics and density functional theory has been used to predict the electronic energy band structure and photo-emission spectrum for graphene-Si system with silicon as a substrate for graphene. The equilibrium geometry of the system after energy minimization is obtained from molecular dynamics simulations. For the stable geometry obtained, density functional theory calculations are employed to determine the energy band structure and dielectric constant of the system. Further the work function of the system which is a direct consequence of photoemission spectrum is calculated from the energy band structure using random phase approximations. © 2014 SPIE.Item Optoelectronic properties of graphene silicon nano-texture(Institute of Electrical and Electronics Engineers Inc., 2014) Brahmanandam, J.; Ajmalghan, M.; Abhilash, R.K.; Roy Mahapatra, D.; Rahman, M.R.; Hegde, G.M.Graphene on silicon with silicon dioxide quantum dots is a promising opto-electronic material. The optical band gap and the corresponding optical conductivity are estimated using the density functional approach with the combination of molecular dynamics. The regular repeating unit cell of graphene silicon nano-texture is identified using the classical molecular dynamics simulations. Electronic calculations predict the optical band gap is around 0.2 eV and the optical conductivity is identified to be 0.3 times the quantum conductance. © 2014 IEEE.Item Heat transfer during quenching of modified and unmodified gravity die-cast A357 cylindrical bars(2006) Prabhu, K.; Hemanna, P.Heat transfer during quenching of chill-cast modified and unmodified A357 Al-Si alloy was examined using a computer-aided cooling curve analysis. Water at 60°C and a vegetable oil (palm oil) were used as quench media. The measured temperatures inside cylindrical probes of the A357 alloy were used as inputs in an inverse heat-conduction model to estimate heat flux transients at the probe/quenchant interface and the surface temperature of the probe in contact with the quench medium. It was observed that modified alloy probes yielded higher cooling rates and heat flux transients. The investigation clearly showed that the heat transfer during quenching depends on the casting history. The increase in the cooling rate and peak heat flux was attributed to the increase in the thermal conductivity of the material on modification melt treatment owing to the change in silicon morphology. Fine and fibrous silicon particles in modified A357 probes increase the conductance of the probe resulting in higher heat transfer rates. This was confirmed by measuring the electrical conductivity of modified samples, which were found to be higher than those of unmodified samples. The ultrasound velocity in the probes decreased on modification. ©ASM International.Item Modification of eutectic silicon in Al-Si alloys(2008) Hegde, S.; Prabhu, K.N.The mechanical properties of Al-Si alloys are strongly related to the size, shape and distribution of eutectic silicon present in the microstructure In order to improve mechanical properties, these alloys are generally subjected to modification melt treatment, which transforms the acicular silicon morphology to fibrous one resulting in a noticeable improvement in elongation and strength. Improper melt treatment procedures, fading and poisoning of modifiers often result in the structure which is far from the desired one. Hence it is essential to assess the effectiveness of melt treatment before pouring. A much investigated reliable thermal analysis technique is generally used for this purpose. The deviation from the standard curve in thermal analysis helps in assessing the level of refinement of the Si structure. In the present review an attempt is made to discuss various aspects of modification, including mechanism, interaction of defects and non-destructive assessment by thermal analysis. © 2008 Springer Science+Business Media, LLC.Item Electrical switching behavior of bulk Si15Te85-xSbx chalcogenide glasses - A study of compositional dependence(2010) Lokesh, R.; Udayashankar, N.K.; Asokan, S.Studies on the electrical switching behavior of melt quenched bulk Si15Te85-xSbx glasses have been undertaken in the composition range (1 ? x ? 10), in order to understand the effect of Sb addition on the electrical switching behavior of Si15Te85-x base glass. It has been observed that all the Si15Te85-xSbx glasses studied exhibit a smooth memory type switching. Further, the switching voltages are found to decrease almost linearly with Sb content, which indicates that the metallicity of the dopant plays a dominant role in this system compared to network connectivity/rigidity. The thickness dependence of switching voltage (Vth) indicates a clear thermal origin for the switching mechanism. The temperature variation of switching voltages reveals that the Si15Te85-xSbx glasses studied have a moderate thermal stability. © 2009 Elsevier B.V. All rights reserved.Item Heat transfer during solidification of chemically modified Al-Si alloys around a copper chill(2011) Prabhu, K.N.; Hegde, S.The solidifying metal/chill contour will significantly affect the boundary heat transfer coefficients, and solidification modellers should be aware of the casting conditions for which the heat transfer coefficients are determined. The previous work carried out on solidification of Al-Si alloys in a metallic mould and solidification against bottom/top chills has shown that modification and chilling have synergetic effect resulting in a significant increase in the heat flux transients at the casting/chill interface. In the present work, the heat transfer during solidification of unmodified and chemically modified Al-Si alloys around a cylindrical copper chill was investigated. Heat flux transients were estimated using lumped heat capacitance method. Lower peak heat flux was obtained with chemically modified alloy. This is in contrast to the results reported for alloys solidifying against chills and in metallic moulds. The chill thermal behaviour and heat transfer to the chill material when surrounded by modified and unmodified alloys were explained on the basis of the decrease in the degree of undercooling in the case of modified alloy as compared to unmodified alloy and the change in contact condition and shrinkage characteristics of the alloy due to the addition of chemical modifiers. © 2011 Institute of Materials, Minerals and Mining.Item Machining characteristics of multiwall-CNT reinforced Al/Al-Si composites using recurrence quantification analysis(2011) Gangadharan, K.V.; Umashankar, K.S.; Ravish; Desai, V.Aluminium (Al)/Aluminium alloy composites are emerging as very promising materials, especially in the fields of aerospace and automotive for their various attractive and technically demanding properties. Discontinuously reinforced aluminium metal matrix composites with reinforcements as nanoparticles of ceramics in general and carbon nanotubes in particular have emerged as the forerunner for a variety of general and special engineering and structural applications. In many of the fields where these materials find applications, machining is invariably required for getting correct geometries, dimensions and surface finish of the components. Hence, establishing the machining characteristics of these materials in terms of the deterministic nature of dynamic signals such as cutting force signals and vibration signals is very important and sought after. Machining process has been understood to be nonlinear and chaotic in nature. In this paper a relatively new technique called Recurrence Plots (RP) and Recurrence Quantification Analysis (RQA), a tool to analyse nonlinear and chaotic systems, is used to study the machining characteristics of cast and powder metallurgy Al and Al-Si alloys (LM6 and LM25), CNT reinforced Al/Al-Si composites produced by powder metallurgy route. Cutting force signals were sensed, acquired and analysed using RQA technique. Determinism (DET), which is one of the variables of RQA, indicates the determinism present in a signal. The values of DET were used to compare the machining characteristics. For all the three materials the deterministic nature of the cutting force signal was highest when reinforced with 0.5 weight percentage CNT, followed by respective base alloys produced by powder metallurgy method and casting route. © 2011 Jordan Journal of Mechanical and Industrial Engineering.Item Effect of chemical modification of Al-Si alloys on thermal diffusivity and contact heat transfer at the casting-chill interface(2012) Prabhu, K.N.; Jayananda; Hegde, S.The heat flow during the unidirectional downward solidification of Al-7Si and Al-12Si alloys was analyzed using thermal analysis technique and inverse modeling. Chills instrumented with thermocouples were brought into contact with a small pool of liquid metal so as to minimize the effect of convection caused by pouring and temperature gradients. Modification melt treatment resulted in an increase in the cooling rate of the solidifying casting near the casting-chill interfacial region. The corresponding interfacial heat flux transients were also found to be higher. The thermal diffusivities of alloys were measured using a laser pulse technique and were found to be higher for modified alloys. However, the increase in the heat flux transients was attributed mainly to the improvement in the casting-chill interfacial thermal contact condition brought about by the decrease in the surface tension of the liquid metal upon the addition of sodium. Copyright © 2012 by ASTM International.Item Fabrication and characterization of thermal evaporated n-Si/ p-ZnTe thin film heterojunction diodes(2013) Rao, K.; Bangera, K.V.; Shivakumar, G.K.The paper reports the fabrication and detailed electrical characterization of thermal evaporated n-Si/p-ZnTe thin film heterojunction diodes. The heterojunction diodes were prepared by depositing ZnTe films on n-Si substrates. The conduction mechanism, barrier height, space charge density and width of the depletion region were determined by I-V and C-V characteristics of the heterojunction diodes. The bandgap and activation energies of n-Si and p-ZnTe were also determined and a theoretical band diagram of n-Si/p-ZnTe heterojunction was drawn based on Anderson's model. © 2012 Elsevier B.V. All rights reserved.Item The effect of the addition of strontium and cerium modifiers on microstructure and mechanical properties of hypereutectic Al-Si (LM30) alloy(ASTM International, 2013) Vijayan, V.; Ravi, M.; Prabhu, K.The present work deals with the melt treatment of LM30 hypereutectic Al-Si alloy using cerium and strontium and the assessment of its effect on microstructure and properties of the alloy. The addition of cerium simultaneously modified both primary and eutectic silicon, leading to an increase in ultimate tensile strength, as well as the wear resistance of the alloy. It was found that the coefficient of thermal expansion of the alloy decreased on modification of the microstructure. © 2013 by ASTM International.